A physical quantity sensor generally includes a sensor portion formed with a substrate such as a semiconductor substrate or a ceramic substrate. The sensor portion is held in a casing made of resin, for example. The sensor portion detects a physical quantity such as acceleration based on a change in an electrical capacitance, a voltage, or the like. If a vibration of the casing is transmitted to the sensor portion, a noise contained in an output of the sensor portion increases. Therefore, there is a need to place a vibration isolation structure between the sensor portion and the casing to reduce a relative vibration between the sensor portion and the casing.
JP-A-2007-212174 discloses a sensor having a resin spring serving as a vibration isolation structure. The resin spring is located between a casing and a sensor portion and supports the sensor portion to the casing to reduce a relative vibration between the casing and the sensor portion. Since the sensor portion is small, the resin spring needs to have a small spring constant. However, it is difficult to form a resin spring having a small spring constant that is accurately adjusted to a derided value. Further, it is difficult to ensure structural strength of the resin spring.